(i) Field of the Invention
This invention relates to the supply of nitrogen of variable purity with the aid of membrane separation. The invention applies more particularly to cases where it is necessary to supply a user whose nitrogen flowrate requirements can vary over a relatively wide flowrate range.
(ii) Description of Related Art
The production of nitrogen by means such as membranes (the term "membrane module" is also often used) or preferential adsorption modules, often referred to in both cases as in situ or "on-site" means, has developed considerably in recent years all over the world, complementing conventional production by cryogenic means, as these means have the following advantages:
excellent reliability of the supply; PA1 low production costs, and PA1 the possibility of supplying, at very attractive costs and in accordance with the applications in question, nitrogens of adjusted purity, sometimes referred to as "impure nitrogens", insofar as the residual oxygen concentration of these nitrogens can vary from several thousands of ppm (parts per million) to several %. PA1 in order to produce nitrogen of reduced purity (e.g. 5% residual oxygen), compressed air brought to a relatively high temperature (e.g. 60.degree. C.) can be used, and PA1 in order to produce nitrogen of high purity (e.g. 1000 ppm residual oxygen), compressed air brought to a relatively low temperature, often close to ambient temperature or lower, can be used. PA1 a) compressed air is passed via the upstream part of a main line in a separating assembly formed by at least two membrane separation lines mounted in parallel, each line including a membrane separator, and wherein at least one of the separating lines can be shut off from the main line by shut-off means, adopting one of the following configurations as the case may be: PA1 a1) when the flowrate requirements of the site are situated at the level of the nominal flowrate, all of the membrane separators of the assembly are in operation, none of the separating lines being shut off; PA1 a2) when the flowrate requirements of the site exceed a predetermined flowrate, at least one of the lines of the separating assembly is shut off, keeping at least one separating line and its membrane separator in operation; PA1 b) the gas obtained at the outlet side of each of the membrane separators in operation in stage a) is advanced to a collector itself connected downstream to a downstream part of the main line; PA1 c) the gas obtained at the permeate side of each of the membrane separators in operation in stage a), enriched with oxygen, is evacuated towards the external atmosphere or advanced to a user station requiring the use of a gas of this kind enriched with oxygen, and PA1 d) the gas originating from the collector is directed along the downstream part of the main line towards at least one user station via a device for controlling the flowrate delivered by the collector, increasing or reducing this flowrate if necessary.
In the particular case of semi-permeable membranes, the principle is that, as a result of the effect of a partial pressure difference on either side of the membrane, a mixture obtained at low pressure enriched with the most permeable components is obtained at the permeate side. A mixture at a pressure close to the supply pressure (of the ingoing mixture) and enriched with the least permeable components is obtained at the membrane outlet (also referred to as the "retentate" side or "discharge" side).
Therefore, semi-permeable membranes having good properties for separating nitrogen with respect to oxygen (selectivity), e.g. of the polyimide or polyaramide type, are used to produce "impure" nitrogen from air, the mixture enriched with oxygen being obtained at the permeate side.
It will therefore be clear that the performances obtained will depend very largely on the conditions of use of the membrane, such as temperature, supply pressure of the membrane, or the content in the supply mixture of the component it is desired to separate at the permeate side.
With respect to temperature, it is known that at a high temperature (e.g. 80.degree. C.), the productivity of the membrane increases, but the O.sub.2 /N.sub.2 selectivity of the membrane deteriorates. In this context, it is often necessary to work under conditions in which the temperature parameter is adjusted.
Still in the case of membranes, a certain number of disadvantages have nevertheless been noted connected to the relatively inflexible nature of this "on-site" production means, particularly with respect to the nitrogen flowrates required and actually used by the user. It is therefore particularly difficult from the outset to design an installation for the "on-site" production of nitrogen at the outset for a user whose flowrate requirements can vary around a nominal value considered average by that user, wherein the flowrate range over which the flowrate can vary is relatively extensive.
Similarly, it is also particularly difficult from the outset to design an installation for the "on-site" production of nitrogen for a user whose flowrate requirements are necessarily going to vary in the course time as a result of changes in production associated with advances or setbacks depending on the circumstances (e.g. leading to the purchase of new furnaces operating with nitrogen).
In order to respond to this difficulty, the solutions existing at present propose, e.g. providing a buffer tank at the user site at a given point when the nitrogen requirements are reduced or increased. This method can only reasonably accommodate moderate variations in consumption, and of short duration.
Another solution consists in providing a reserve of liquid nitrogen produced by cryogenic means for increases in consumption, this solution of course having the disadvantage of its additional cost.
Other more complicated solutions generally assume a modification of the source of processed air, this having negative effects with respect to capital investment or the response time of the equipment when the user station has to move rapidly from a nominal flowrate to an increased or reduced flowrate.
U.S. Pat. No. 4,806,132 can therefore be cited by way of example, relating to the case where the membrane (particularly in the case of nitrogen production) must be able if necessary to produce a lower flowrate than the nominal cruising flowrate used, wherein the initial purity of the nitrogen must be maintained or can undergo a certain deterioration.
The document cites the example of an "on-site" nitrogen consumption which could be decreased by 30% relative to a nominal flowrate. The solution proposed by this document is to modify the air source by reducing the flowrate of the compressor and the pressure of the air supplied, by adjusting parameters, such as the opening of valves downstream of the compressor or of the drive variables of this compressor. This method of operation therefore necessitates a modification of the air source, this constituting a significant complication and having consequences from the point of view of instrumentation and therefore capital investment.
The European patent EP-A-517 570 can also be cited by way of example, the aim of which is to be able to adapt to an increasing or reduced consumption, if necessary also with a variation from the point of view of the nitrogen purity required. The solution proposed if overconsumption occurs on site over a wide flowrate range consists in increasing the flowrate of processed air (e.g. by increasing the number of compressors or by having overdesigned the compressor) and in parallel by increasing the permeability of the membrane by playing on the temperature. This solution therefore has disadvantages from the point of view of capital investment, an increase in instrumentation and an increase in the response time of the equipment (particularly to temperature).
Finally, U.S. Pat. No. 4,397,661 can also be cited by way of example, this document envisaging a different case as it describes the processing and separation of a gaseous mixture with the aid of an assembly of membranes in which, in contrast to the preceding cases, it is the flowrate of the ingoing mixture that can be varied, as is the case with the separation of hydrogen from an H.sub.2 /N.sub.2 /CH.sub.4 /Ar mixture produced by any installation such as that of a refinery. In addition, the element it is desired to separate in this case is obtained at the permeate side of the membrane. The solution proposed in this document assumes the use of a variable membrane surface area. The higher the flowrate to be processed, the higher the number of membranes used (in parallel), while endeavouring to keep the concentration of the permeable element at the permeate side of the membranes constant.
The permeate of each of the membranes is thus connected to a common recovery line.